Protection against pathogenic bacteria and viruses (as well as other pathogenic microbes, including parasites and fungi) can be conferred on an individual in three ways: a) passive immunization—direct infusion of antibodies raised against a specific organism, b) prior exposure to the micro-organism itself, or c) prophylactic vaccination against the organism. In the two latter cases, the exposed individual's adaptive immune system is activated at both the humoral and cellular levels. Humoral responses involve production of antibodies against the pathogen (or a component of it) by circulating B lymphocytes; the antibodies bind to the organism, thus tagging it for destruction or removal by other elements of the immune system. Cellular responses are complex, and involve activation of many different cell types within the host's immune system (including the innate immune system components); these cells are then either directly or indirectly involved in the destruction or removal of the pathogen, or host cells that may already be infected by the pathogen. For general background on vaccination one may consult, for example, Charles A. Janeway et al., Immunobiology (6th ed. 2004).
A key step in the immunization process is to ensure that the antigen is delivered to a tissue that contains antigen presenting cells (APCs). These cells are responsible for acquiring immunogenic components of potential pathogens, and displaying them on their cell surface in such a way that they interact successfully with key components of the immune system to mount the robust humoral and/or cellular response required for protective immunity.
The density of APCs in muscle tissue is considerably lower than that in the epidermal layer of the skin. However, vaccines are normally administered via direct injection into muscle, a procedure that has been dictated more by convenience for the health care practitioner than by the role that muscle tissue plays in the immune system. The pain and bleeding that often results from damage to blood vessels (muscle being highly vascularized) can result in poor patient compliance.
The epidermal layer of the skin is a convenient tissue for antigen delivery since it contains neither nerves nor blood vessels and it is rich in a specialized type of APC, the Langerhans cell. Delivery of vaccine components to this tissue is often referred to as “transcutaneous” immunization. Transcutaneous immunization may be achieved by use of ordinary needles in an intradermal mode of delivery. It is commonly carried out using adjuvants. “Transcutaneous immunization (TCI) is a new method of vaccination that utilizes a topical application of an adjuvant and vaccine antigen to intact skin to induce an immune response.” Gregory M. Glenn et al., “Transcutaneous immunization: a human vaccine delivery strategy using a patch,” Nature Medicine, vol. 6, 1403-1406 (2000). See also U.S. Published Patent Application No. 2007/0088248.
Arrays of microneedles were proposed as a way of administering drugs through the skin in the 1970s, for example in expired U.S. Pat. No. 3,964,482. Microneedle arrays can facilitate the passage of drugs through or into human skin and other biological membranes in circumstances where ordinary transdermal or topical administration is inadequate. Microneedle arrays can also be used to sample fluids found in the vicinity of a biological membrane such as interstitial fluid, which is then tested for the presence of biomarkers.
Despite much initial work on fabricating microneedle arrays in silicon or metals, there are significant advantages to polymeric arrays. U.S. Pat. No. 6,451,240 discloses some methods of manufacturing polymeric microneedle arrays. Arrays made primarily of biodegradable polymers have some advantages. U.S. Pat. No. 6,945,952 and U.S. Published Patent Applications Nos. 2002/0082543 and 2005/0197308 have some discussion of microneedle arrays made of biodegradable polymers.
Microneedle arrays are believed to have advantages for vaccine delivery. See, for example, James A. Matriano et al., “Macroflux® Microprojection Array Patch Technology: A New and Efficient Approach for Intracutaneous Immunization,” Pharmaceutical Research, vol. 19, p. 63 (2002).
There is therefore a need for an effective means of delivering vaccines via microneedles and of making use of the advantages of microneedle delivery for vaccines.